Morning walk is . . . . . . . . tonic for . . . . . . . . weak, cure for the sick and . . . . . . . . exercise for . . . . . . . . healthy.

In the given sentence, the word, 'tonic' means 'a medicinal substance taken to give a feeling of vigour or well-being'. There are many 'tonics' in the world. 'Morning walk' is mentioned as one of them. As it refers to 'one of many' tonics, an indefinite article is used before the word, 'tonic'. The word 'exercise' is also used similarly. So, before the words, 'tonic' and 'exercise', an indefinite article is to be used.
The choice between the indefinite articles, 'a' and 'an' is determined by the sound. Before a word beginning with a vowel sound, 'an' is used. Before a word beginning with a consonant sound, 'a' is used. The noun, 'tonic' begins with a consonant sound 't'. The noun 'exercise' begins with a vowel sound, 'e'. Hence, we use 'a' before 'tonic' (a tonic) and 'an' before 'exercise' (an exercise).
The words, 'weak' and 'healthy' are adjectives. However, they are also sometimes used as nouns to refer to the entire class of people who are 'weak' and 'healthy' respectively. Hence, the definite article, 'the' is used before both these nouns.